For vehicle windscreens in particular, there is a great demand for heatable designs, it being required that the heating as such is as invisible or unobtrusive as possible. Therefore, a heatable transparent coating of the panes is increasingly in demand.
A general problem of heatable coatings with low light absorption is their still relatively high sheet resistance, which at least in the case of large dimensions of the window to be heated or long current paths requires a high operating voltage, which is at least higher than the conventional voltages of the electrical systems in vehicles. If it were desired to lower the sheet resistance, with the previously known systems of layers this would entail a reduction in the transmission of visible light, since the conductive layers would have to be thicker.
For these technical reasons, wire-heated panes, which can be readily fed with the conventional vehicle voltage, are currently still fitted with preference. However, these laminated windows with inlaid heating areas comprising very thin wires are not accepted by all customers.
The patent DE 1 256 812 B1 describes a glass sheet which can be heated by means of an electrically conductive layer of metal or metal oxide applied over its surface area. This publication with application priority from 1963 is based on a very high sheet resistance of 200 Ω/unit of surface area. In order nevertheless to heat this layer homogeneously with a relatively low voltage by means of two lateral low-impedance bus bars, narrow, printed, low-impedance comb electrodes are provided, extending from said bus bars over the entire viewing area of the pane. These electrodes mesh with one another with alternating polarity. They only end in each case shortly before the bus bar respectively lying opposite. This does indeed achieve the effect that, transversely to the longitudinal extent of the individual lines of the comb electrodes, the heating current only has to cover relatively short paths within the layer.
Mentioned there as an advantage in comparison with windows which are only heated with the aid of printed narrow heating conductors is the homogeneous heating output with relatively great mutual distances between the comb electrodes.
However, the said lines, resembling shading, disturb the view through the window and the visual appearance of the main viewing area of the pane configured in this way. The optical advantage of a transparent heating layer remains unused. This window is only intended as a rear window for automobiles. Even now, it should not be used as a windscreen, since they must not have anything impeding the view, at least in a standardized A viewing area, as it is known.
A further problem with coatings for heating purposes may be caused by factors such as that they cannot be applied homogeneously over the entire surface area of the transparent pane but instead have to be provided with one or more interruptions in them, known as communication windows, which impair the flow of the heating current and possibly lead to the formation of hot spots (instances of local overheating) at their edges. Such communication windows serve the purpose of making the coating locally more transmissive for certain information flows or signals, whereas it intrinsically reflects shortwave or infrared rays.
For leading the heating current in and out of such coatings, at least one pair of electrodes (in strip form) or collecting conductors (also known as bus bars) is provided, intended to lead the currents into the surface of the layers as uniformly as possible and distribute them widely. In the case of vehicle windows, which are appreciably wider than they are high, the bus bars usually lie along the longer edges (at the top and bottom in the fitted position) of the window, so that the heating current can flow along the shorter path over the height of the window. At the same time, the communication windows mentioned usually lie at the upper edge of the window, where they extend over a width of several centimeters.
The document WO 00/72 635 A1 describes a transparent substrate with an IR-reflective coating and a communication window produced by removing or omitting the coating over a surface area.
Obviously, any communication window that changes the homogeneity of the coating constitutes a disturbance of the current flows. Local temperature peaks (hot spots) occur and may lead to damaging of the substrate (thermal stresses) and of the coating itself. This is not only the case when the coating is omitted over a large area but also when the communication window is formed by a greater or lesser number of individual, discrete slits. These also constitute a notable increase in the sheet resistance in the area concerned and at the same time likewise cause the hot spots mentioned.
As a measure for reducing the disturbing effect of the large-area communication window, the last-mentioned document proposes providing at the edge of said window an electrically conductive band that has a very much lower ohmic resistance per unit area than the heating layer. It is intended to make the currents bypass the cutout. With preference, a communication window is completely surrounded by such a band. The band can be produced by printing on and baking a conductive screen printing paste containing silver. It may, however, also be provided by applying an electrically conductive lacquer or by placing on a metallic strip. In all cases, an electrically conducting connection of the band to the coating is of course functionally necessary.
The band may be optically masked by overlaying an opaque, electrically nonconductive masking strip, for example of black enamel. Such masking strips generally consist of a nonconductive, black-coloured material that can be baked (screen printing paste). Infrared radiation is not reflected by this material but absorbed.
WO 03/024 155 A2 discloses a relevant transparent window with a heatable coating in which an operating voltage of at most 42 V is specified but which also attempts to solve the problem of hot spots at the edges of a communication window. In general, a number of different voltage levels are used, a lower voltage being applied to current paths that have been shortened (for example because of the communication window), in order to avoid local overheating. Specifically, the region of the communication window is left in the heatable surface area by placing a separate bus bar between the communication window and the bus bar on the other side.
Furthermore, DE 36 44 297 A1 discloses many examples of subdividing heatable coatings of a vehicle windscreen. Subdivisions can accordingly be provided by leaving portions without layers over a surface area and/or by incisions being made mechanically or by laser radiation. They serve for the selective setting and directing of a current flow within the coated surface area and are intended to ensure a current density that is as uniform as possible in the surface areas concerned.
WO 2004/032569 A2 discloses a further design of a transparent pane with a heatable coating, which likewise seeks to achieve a homogeneous heating output in the surface area by separating lines introduced into the coating.
DE 29 36 398 A1 is concerned with measures for preventing current peaks at the transition from the bus bars to the coating in a transparent window with a heatable coating. It is generally endeavoured to reduce the abrupt difference in resistance between the coating and the bus bars by using materials or formations of higher impedance for the latter, or else by means of intermediate resistances. For the coating, sheet resistances of between 1 and 10 ohms per unit area are specified there. In one or more solution variants described there, the edge of each bus bar facing the respectively opposite bus bar is of a wavy form. This is intended to avoid the formation of peaks directed towards the coating for heating purposes. With this approach, a notable lengthening of the transitional line between the bus bar and the coating is sought, and consequently a reduction in the current density at this transition. However, all these measures appear to be little suited for allowing the heatable layer to be fed with a relatively low voltage.
It is also known to provide what are known as grid or comb electrodes on the light incidence side of photovoltaic solar cells (see for example WO 03/075 351 A1). They are often produced by screen printing and comprise a bus bar arranged at the edge of the solar cell and a plurality of very narrow prongs, which extend from the bus bar over the surface area of the solar cell. They make it possible for the photovoltaic voltage that is applied to the two sides representing the surfaces of the absorber or between the comb electrode on the front side and the metallic/full-area rear electrode to be picked up over the surface area, without greatly reducing the light that enters the absorber.
DE 197 02 448 A1 discloses a heatable mirror, on the glass body of which two conductor tracks or electrodes formed in the manner of a comb and interlocking each other are applied with a PTC coating covering them and filling the intermediate spaces between the prongs of the comb. Here, however, the problem of making the coating visually unobtrusive does not arise, because the conductor tracks and the heating layer can lie behind the mirror layer.
DE 198 32 228 A1 describes a vehicle window with an electrically conductive and optically transparent coating that is used as an antenna. High-frequency radio signals are picked up purely capacitively from the antenna layer with the aid of a coupling electrode, which comprises a number of thin interconnected wires that are arranged parallel to one another at a great distance in comparison with their diameter and extend from the edge into the viewing area of the window, where they end blind. There is no galvanic coupling between the coating and these wires, since they are in each case arranged in different planes of the laminated window.
The patent DE 10 2004 050 158 B3 describes a transparent pane with a heatable coating.
The bus bars, already mentioned several times, may be produced both by (screen) printing, before or after the layer is applied to the window, or by soldering on thin metal-band strips, preferably made of (tin-plated) copper. There are also combinations of printed and metal-band bus bars (see for example DE 198 29 151 C1). Although the bus bars are usually configured in the form of a narrow band, they are opaque. For optical reasons, they are therefore respectively arranged in the vicinity of the outer edge of the transparent windows concerned. They can usually be masked by opaque edge coatings (usually likewise produced by screen printing). The communication windows mentioned may also be masked by these edge coatings, as long as the latter are sufficiently transmissive for the radiation that is to be transmitted.
In the case of common vehicle windscreens, these opaque coatings are configured as frames which, as a further function, shield the adhesive bond between the window and the vehicle body against UV rays. These frames circumscribe the viewing area of the windows. In the case of windscreens, a distinction is made furthermore between an A viewing area, in the middle of the area of the window, in which there must not be any viewing impediments (for example colourations, wires or instances of damage), and the B viewing area, lying closer to the edge.